The present invention relates to fluid coupling devices of the type including both a fluid operating chamber and a fluid reservoir chamber, and valve means operable to control the quantity of fluid in the operating chamber.
Although the present invention may be used advantageously in fluid coupling devices having various configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Fluid coupling devices of the viscous shear type have been popular for many years for driving engine cooling fans, primarily because their use results in a substantial saving of engine horsepower, resulting from the fact that the coupling operates in the engaged position only when cooling is needed, but operates in a disengaged, relatively lower speed condition when little or no cooling is required.
For many years, one of the problems associated with fluid coupling devices used to drive radiator cooling fans has been the phenomenon known as "morning sickness". A typical fluid coupling device includes one or more fill openings, through which viscous fluid flows from the reservoir into the operating chamber, and one or more discharge openings, through which fluid is pumped from the operating chamber into the reservoir. After the coupling device has been operating for a period of time, and the engine is turned off, the coupling device ceases rotation. When the device stops rotating, the orientation of the fill and discharge openings are completely random.
If the discharge opening is below the fluid level in the reservoir, of a typical coupling device, a certain amount of fluid will leak from the reservoir, back through the discharge opening, into the operating chamber.
When operation of the fan drive begins again, after a long period of time (for example, the next morning), there is a substantial amount of fluid in the operating chamber. As a result, although the engine is cold and no cooling is required, the coupling device will operate in the engaged condition for a period of time, until most of the fluid in the operating chamber is pumped back into the reservoir chamber. Unfortunately, such engaged operation results in undesirable noise of the fan being driven when it is not required. This phenomenon, most common and perhaps most objectionable in the morning, has been referred to as "morning sickness".
A number of different solutions have been tried over the years, with the main objective being faster pumpout of fluid which does leak back into the operating chamber, from the reservoir chamber, during a period of prolonged shutdown of the coupling device. Certain of these attempted solutions have been helpful in reducing the "dump time", but have not really solved the fundamental problem, i.e., the drain back of fluid into the operating chamber.
A different type of solution is illustrated in U.S. Pat. No. 4,312,433, assigned to the assignee of the present invention. In the cited patent, the cover of the fluid coupling has a counterbore where the discharge passage enters the reservoir chamber, and a tubular member ( a "standpipe") is received in the counterbore. The standpipe typically extends to a location near the rotational axis of the coupling device, such that its radially innermost end is likely to be above the fluid level of the fluid in the reservoir chamber. The standpipe concept has been generally satisfactory in reducing bleedback and "morning sickness", but requires a substantially more complex and expensive cover casting, and more machining. In addition, with the typical standpipe arrangement, the coupling device may still have certain orientations which permit fluid bleedback, from the reservoir into the operating chamber.